Most aircraft have both a "flight control" and a "trim control" system. The flight control system is used to maneuver the aircraft during flight, i.e. to make course changes, etc. whereas the trim control system is used to adjust the flying characteristics of the aircraft. In large aircraft, both systems are fluid pressure operated. Controls operated by the pilot control the application of fluid pressure to an actuator that is attached to the control surface being adjusted or moved. These control surfaces may include ailerons, the horizontal stabilizer and the vertical stabilizer.
Most flight control systems used today are modulating type systems in which some fluid pressure is always applied to the actuator attached to the control surface. Positional changes are made to the control surface by altering the pressure differential applied to the associated actuator. As a result these systems are considered to be continuous duty systems since during flight, the components of the system are continually stressed by at least a minimum fluid pressure level. Many of these systems include feedback arrangements to provide more precise control and in some instances the control system forms part of an autopilot system so that fully automated flight can be obtained.
The trim control system of an aircraft on the other hand, is considered an intermittent duty system, since it is used to periodically adjust the aircraft's flight characteristics. The adjustments may be made in order to compensate for aircraft flight control loads that occur in response to changes in speed, attitude, load and/or changes in drag resulting from actuating the landing gear, flaps and other external aircraft surfaces. The system typically controls the horizontal stabilizer; slight adjustments to its position change the overall flying attitude of the plane. Since in the past it was believed that adjustments to these control surfaces were done only infrequently during flight, the control systems for controlling these surfaces was not of the modulating type.
A typical control system for a horizontal stabilizer is simple in nature and can be termed as a "bang-bang"system. When a change in the position of the control surface is desired, fluid pressure is applied to the associated actuatcr to cause its rotation and attendant movement in the surface. After the movement is completed, the actuatcr is completely depressurized. A mechanical brake is used to lock its position. In a typical control system for a large aircraft, the pressure applied to the actuator can be in the range of 3,000 psi. As a result, the components forming part of the control system are abruptly cycled between 0 and 3,000 psi whenever the system is activated. This large and sudden pressure fluctuation places the components under substantial stress.
It has been found recently, that contrary to what was believed in the past, in some aircraft the trim control system is activated by the pilot at a higher than expected frequency. This is especially true in large military aircraft that are capable of being refueled in flight. As the aircraft is being refueled, the pilot must make continual trim changes to the aircraft to compensate for the changes in aircraft weight and center of gravity that occur as the fuel is being delivered. Changes in aircraft weight that cause changes in flight characteristics also occur as a result of fuel consumption. Failures in the trim control system have been occurring at a rate that is undesirable. It is also believed that simply replacing a conventional trim control system with a sophisticated modulating system as is used for flight control system would be prohibitably expensive.